Thursday, February 19, 2009

Why do humans reproduce sexually?

The evolution of sexual reproduction is not widely discussed or questioned in non-academic circles, it is taken as a given that humans, and the majority of species throughout time, reproduce through the fusion of gametes. Yet it is uncertain whether this is the most efficient and secure way of continuation of our species. Asexual reproduction is accepted as the most rapid way to expand a population from one generation to the next, as described by John Maynard Smith,[1] so why do humans not reproduce in such a way? Many theories have been put forth for all sexually reproducing species, some only apply to certain species and many explanations intertwine with one another. One justification is the increased capacity for genetic variation to occur, which links closely to the hypothesis that sexual reproduction is primarily in order to resist parasites. Both are countered by the suggestion that the occurrence is necessary to remove deleterious genes or is an adaptation to substantial chromosomal damage and mutation. These theories have been combined along with my own thoughts in order to ascertain in my own mind why Homo sapiens have evolved to reproduce sexually.

Charles Darwin, the father of modern evolutionary thinking, stated that the advantage of sex is “the offspring of two individuals, especially if their progenitors have been subjected to very different conditions, have a great advantage in weight, constitutional vigour and fertility over the self fertilized offspring from either one of the same parents.”[2] Thus he suggests that a genetically superior individual is created by two sexes, rather than asexually. This supports the hypothesis of repair and complementation, or hybrid vigour, where genetic recombination and outcrossing are adaptations to chromosome damage and mutation. It is suggested that recombination during meiosis (and thus sexual reproduction) is primarily required in order to repair damaged DNA. However, this contradicts the widely accepted suggestion that recombination evolved primarily to increase genetic variation between parent and offspring. Therefore if Darwin were still to maintain the significance of recombination in the evolution of sexual reproduction then his theory would have to move more to support the view that sex chiefly occurs in humans in order to maintain variation. Nevertheless, as will be pointed out later, it is questionable how vital variation is to specifically the human race. In addition, it has been put forth by the likes of Elshel that recombination disrupts positive combinations of genes more often that it creates them.[3] Consequently there are evident flaws in Darwin’s suggestion and the repair and complementation hypothesis, particularly relating to the original purpose and importance of recombination during meiosis.

Darwin’s thought that the birth of a genetically stronger human is the main reason for the evolution of sex points to the ability to survive, and thus can be linked with the Red Queen Hypothesis,[4] where the fundamental advantage of sexual reproduction over asexual reproduction is the resistance to parasites. This hypothesises that the underlying purpose of sexual reproduction is to allow enough genetic variation for evolution to occur quickly enough to maintain a population despite the evolution of its parasites. This seems to suggest that sexual reproduction is not only an outcome of evolution, but also the cause of continued evolution once it manifested itself in our ancestors. Ridley’s theory is currently the most dominant in terms of support from leading academics, and can be applied directly to human evolution, convincingly arguing why asexual reproduction does not occur in Homo sapiens.

The basic purpose of natural selection seems to be survival. This depends on the ability to adapt to a change in the environment, usually through an increase in variation and the size of a species’ gene pool. Thus this statement seems to relate to the genetic variation that is supposedly necessary in the Red Queen Hypothesis. This opinion is also directly supported by Weismann’s original theory that the advantage of sexual reproduction is variation between siblings.[5] Ghiselin supported this with the Tangled Bank Hypothesis[6] that suggests an assorted population of siblings may be able to extract more food from their environment. However, this poorly applies to humans due to the relatively few siblings that are produced throughout our evolution, along with the fact that Homo sapiens are a highly homogenous species – DNA between individuals is very similar. Sex may also be necessary due to the greater ability it has over asexual reproduction to produce new genotypes. This again puts emphasis on the influence of recombination, that allows for two advantageous alleles on the same chromosome of different individuals of a population to eventually combine. In an asexual population it would take a mutation for a chromosome to gain the two alleles. However, this theory depends on group selection, a weaker selective force than natural selection, as individual fitness is not the motivating factor, but benefit to the group is. This is due to the Two Fold Cost of Sex, put forth by John Maynard Smith,[7] whose theory states that asexual reproduction would bare more young than sexual reproduction. In addition the two sexes have to find one another and are only attracted to certain features. Therefore it is unlikely advantageous alleles would come together within a population. However, the Two Fold Cost of Sex has been countered by the fact that species that can produce by both means choose to reproduce sexually over asexually whenever they can. This points to an overall advantage of sexual reproduction in terms of the central theme of survival. Yet, if the main purpose of sexual reproduction in humans is simply survival, then why has the evolution of feeling such as love occurred? It may be suggested that love has the purpose of leading to reproduction, yet without love there would be more indiscriminate mating within the species, increasing variation within the human population. Therefore survival through variation of a species may not be the main purpose of sexual reproduction, drawing more support towards the Red Queen Hypothesis.

Whereas the Tangled Bank Hypothesis refers to recombination of advantageous genes, there is also a theory that the purpose of sexual reproduction is to remove harmful genes caused by mutation through natural selection. Kondrashov’s theory, the Deterministic Mutation Hypothesis,[8] suggests that because each mutations is only slightly damaging, they will accumulate from one generation to the next until the individuals that have gained many mutations die. However, he writes that there must be at least one adverse mutation per generation, per genome, for the theory to apply. This is feasible for humans because of the “genomic deleterious mutation rate … [of] at least 3”[9] in Homo sapiens.

All the above-mentioned approaches hold some weight in determining the reasons for the evolution of sex in humans. The repair and complementation hypothesis is said to result in a human being that has a greater chance of survival, along with the theory that genetic variation is the core reason for sexual reproduction. The Red Queen Hypothesis primarily centres on the ability of sex to allow for greater variation, so other than the Deterministic Mutation Hypothesis, variation is a common theme throughout. This is also the backbone of Darwin’s theory of Natural Selection, where variation leads to individuals with advantageous genes passing these characteristics onto their offspring. Despite the more rapid reproductive potential of agamogenesis, variation, and thus a greater chance of survival, is evidently the advantage that sexual reproduction brings to Homo sapiens.

6 comments:

An interesting topic. I think that the title is a poor choice, though. Humans reproduce sexually because their ancestors did, is the only valid anwer to the question you pose in your title. I know that it is tempting to look at questions like this from a human viewpoint (we ARE humans after all) but it isn't really relevant whether it is advantageous to humans to reproduce sexually. The "decision" for sexual reproduction only was made in our ancestors. So all discussion of number of offspring or why humans feel love for significant others is irrelevant. It's not as if we had a choice.I don't really get why you think that for sexual reproduction to have an advantage over asexual reproduction you need group selection. If two independent allels provide an advantage on their own, they'll probably be selected for. Individuals with a combination of both will have an even bigger advantage and they'll pass on both advantageous allels. So sexual recombination will allow an even faster spreading of the advantageous allels and make it ever more likely that they'll be combined in offspring.At least that's what I think.

It should be pointed out that the 'Red Queen hypothesis' is not Matt Ridley's he's merely a science journalist writing up work in the field undertaken by others.He's also the ex-Chairman of the failed bank Northern Rock - something for which he's entitled to full credit.

Sabine is right in drawing attention to the "decision" to reproduce sexually having been made way back in human phylogeny, but there are examples of lineages abandoning sexual reproduction, most famously the parthenogenetic lizards.

I'm most puzzled by your statement "However, the Two Fold Cost of Sex has been countered by the fact that species that can produce by both means choose to reproduce sexually over asexually whenever they can."

This is simply not true. Take aphids. They use asexual reproduction on arriving at a host and resort to sexual reproduction when setting off to colonise new hosts. Does that not suggest that clonal reproduction is a good thing when the environment is essentially unchanging, while recombination is a good thing when the environment is changeable? And that environment would certainly includes pathogens.

As for love, you seem to see it in terms of "leading to reproduction". I'd suggest that it leads to parenting, and could thus easily be selected for.

I'm glad you cite Ghiselin, who in my opinion is far too often forgotten.

"Love" is not a valid limitation on "indiscriminate mating". You're assuming that "love = monogamy". This is a cultural bias of social indoctrination, not a true state of animal mating reality. Monogamous animals are a myth. Even the most celebrated so-called monogamous pairings between animals are shown to not necessarily be maintained between seasons, and still show plentiful examples of "side flings". Love is therefore not an example of an evolutionary limiting mechanism on mating.